Synchronous reluctance motors are motors in which torque is generated by resorting to a configuration in which magnetoresistance varies in the rotation direction of the rotor, through formation of slit-like flux barriers in the rotor core. Compared with induction motors (induction machines), synchronous reluctance motors are advantageous in that, for instance, there occurs no secondary copper loss in the rotor. The use of synchronous reluctance motors in factories, air conditioners, automobiles and the like has thus earned attention.
Synchronous reluctance motors, however, are usually considered to exhibit poor power factors, and further improvements on this score are required in order to be used in the above applications. The output torque of a synchronous reluctance motor, referred to as reluctance torque T, can be expressed according to Expression (1) below, where the generation principle is the abovementioned difference in the magnetoresistance in the rotation direction of the rotor.T=Pn(Ld−Lq)idiq  (1)
In Expression (1), Pn is the number of pole pairs, Ld and Lq are d-axis and q-axis inductances, respectively, and id and iq are d-axis and q-axis currents, respectively.
Expression (1) reveals that increasing the difference Ld−Lq between d-axis and q-axis inductances is effective in terms of increasing efficiency by increasing the torque per current in the synchronous reluctance motor.
Moreover, as is known, it suffices to increase a ratio Ld/Lq between the d-axis and q-axis inductances to achieve an increase in the power factor of synchronous reluctance motors.
The value of the ratio Ld/Lq of the d-axis and q-axis inductances is generally referred to as salient pole ratio.
Thus, in order to increase the difference Ld−Lq between d-axis and q-axis inductances or the salient pole ratio Ld/Lq in a synchronous reluctance motor, configurations have been adopted in which a plurality of layers of slits referred to as flux barriers is provided in a rotor core, so that, as a result, d-axis magnetic paths are formed through which magnetic flux flows readily in a direction along the plurality of layers of slits, while magnetoresistance is increased in q-axis magnetic paths that cross the plurality of layers of slits.
In addition, technologies including the following which rely on the above flux barrier structure as a basic structure have been proposed in order to increase torque and enhance efficiency.
On the premise of controlling the stator so that magnetic flux that flows from the stator into the rotor does so towards the rotor center, a synchronous reluctance motor has been proposed for instance in which, in a rotor core obtained through stacking, in the rotor axial direction, of core sheets in which core layers (strips) are disposed so as to bulge towards the center, the radial-direction width of the core layers is set to be wider in central core layers of the rotor than in outer core layers of the rotor, and the radial-direction width of the slits is set to be wider in central slits of the rotor than in outer slits of the rotor or to be the same therebetween (see, for instance, PTL 1 (in particular, paragraphs [0002] to [0017], FIG. 2)).
In this synchronous reluctance motor, the core layers are set to be thicker on the rotor center side, where the magnetic flux flowing from the stator concentrates, and the torque of the motor can therefore be enhanced without the occurrence of magnetic saturation.
Another synchronous reluctance motor has for instance been proposed in which torque ripple can be reduced by setting the widths of a plurality of core layers (divided magnetic paths), which generate one flux barrier (magnetic path set), to be narrower at the center-side portion and outermost portion and to be wider at the middle portion within that flux barrier group (see, for instance, PTL 2 (in particular, paragraph [0021], FIG. 4)).